Using iptables to block ALL outgoing traffic from one NIC?
With iptables -A OUTPUT -o eth1 -j DROP
you can drop all outgoing traffic on interface eth1.
You'll probably also want to drop all forwarded traffic using iptables -A FORWARD -o eth1 -j DROP.
To drop all the outgoing traffic on eth1
iptables -I OUTPUT -o eth1 -j DROP
will insert a rule at the begining of the OUTPUT chain to drop all outgoing traffic.